If FarmVille isn't all that great of a game, why do people play it? It's the same reason people go on vacations they're not really looking forward to. The "sunk cost fallacy" is a strong, almost primordial thing, and it helps to understand it.

You Are Not So Smart, the go-to blog for understanding why we all do silly things, tackles the psychological roots of sunk cost. Put simply, it's the feeling of "I've already spent X, so I'd better spend Y to salvage it." David McRaney's post makes good sense of how FarmVille is a master manipulator of the sunk cost emotion, but shows how it's hardly contained to pastimes and cheap addictions:

Hal Arkes and Catehrine Blumer created an experiment in 1985 which demonstrated your tendency to go fuzzy when sunk costs come along. They asked subjects to assume they had spent $100 on a ticket for a ski trip in Michigan, but soon after found a better ski trip in Wisconsin for $50 and bought a ticket for this trip too. They then asked the people in the study to imagine they learned the two trips overlapped and the tickets couldn't be refunded or resold. Which one do you think they chose, the $100 good vacation, or the $50 great one?

Over half of the people in the study went with the more expensive trip. It may not have promised to be as fun, but the loss seemed greater ... The fallacy prevents you from realizing the best choice is to do whatever promises the better experience in the future, not which negates the feeling of loss in the past.

When have you most regretted your habits of throwing good time, money, or energy after bad?